Unstable by Design

by Dr. Richard Stimson

in The Kitty Hawk Years

The Wright Flyer has very unstable flying characteristics because of its canard design (elevator in front). Even the Wright Brothers had trouble flying the machine and they had plenty of practice.

At Kitty Hawk Wilbur stalled the machine on his first attempt to fly on December 14, 1903. Three days later Wilbur and Orville were able to fly, but it was an undulating ride as the machine pitched up and down.

On the fourth flight that day Wilbur flew a distance of over 800 feet, but the machine was capable of flying all the way to the village of Kitty Hawk except that it pitched down and hit the sand which ended the flight prematurely.

The Wrights had good reason to select the canard design. They feared the life-threatening stall that killed Lilienthal in 1896. The stall is a condition when the wing ceases to have lift at a high angle of attack and the airplane dives and rolls.

If one is flying close to the ground as the Wrights were, there is no time to recover from a stall before crashing into the ground.

The canard design mitigates this condition. The small wing of the canard design stalls first while the main wing still retains some lift. The machine tends to sink flatly instead of diving steeply.

Another advantage of the canard is that the front elevator acts as a shock-absorber bumper. When Wilbur stalled the machine on December 14th, he broke the front elevator but he himself was unharmed.

The Wrights did not appreciate the aerodynamic forces that produced the instability of the canard design. They were well aware of the four basic forces of flight – lift, weight, thrust and drag – and that flight involved a balancing act among these forces.

What they did not take into account was how this balancing of forces changed when an airplane was flying. Changes in direction caused either by the pilot or the wind introduce rotational torques or moments that act upon the airplane.

Every lifting surface on an airplane can be considered to have a point where the lift and drag forces are focused. Aeronautical engineers call this the “aerodynamic center” and it is usually located approximately one-fourth back from the leading age of a lifting surface.

The total effect of all lifting surfaces on an airplane is centered at a point called the “neutral point.”

Without getting too detailed, here is what all this means.

An inherently stable airplane is one in which the “center of gravity” is located in front of the neutral point.

An inherently unstable airplane is one in which the “center of gravity” is located behind the “neutral point.” This is the case of the Wright Flyer with its canard design.

When the Wright Flyer is in flight, a rise of the nose, increases its lift and the “pitching moment” reinforces the upward movement of the nose.

In a stable configuration, the “pitching moment” resists the upward pitching movement.

An unstable machine requires the pilot to continually make adjustments to maintain pitch. The Wrights desired a machine that was unstable because they wanted to control it. They got more than they wished for.

In 1904 the problem of undulating flight at Huffman Prairie was causing them a lot of problems. They tried changing the center of gravity by moving the pilot position and the engine to the rear. That made things worse. They then moved the center of gravity forward by adding 20 pounds of ballast beneath the front elevator. That helped some

In 1905, they were still having undulation problems. The situation came to a head on July 14 when Orville lost control of the elevator and crashed at a speed of 30 mph. He was lucky not to have been badly injured. When he hit the ground, he was thrown through a broken section of the top wing and ended up in a tangle of the broken elevator dazed and bruised.

They then modified the machine by enlarging the elevator and moving it some four feet further out from the wing. The changes made a significant improvement in the flying ability of the airplane.

On October 5, Wilbur flew thirty circles over Huffman Prairie, landing only when his plane ran out of gas. He had been in the air over 39 minutes. It was the world’s first practical airplane.

The Wrights can’t be faulted for not understanding all the dynamic forces that were present while flying. It would be another two decades before aeronautical engineers began to understand them.

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